1. Field of the Invention
The present invention relates to a high-frequency power supply device which supplies high-frequency power to a load, and more particularly relates to a high-frequency power supply device which is suitable for use as a power supply in plasma generating apparatus, laser oscillators and the like.
2. Description of the Related Art
As shown in FIG. 22, high-frequency power supply devices which are used as power supplies in plasma generating apparatus, laser oscillators and the like are basically constructed from an oscillator to output a high-frequency signal of a specified frequency, an amplifier which amplifies the output of the oscillator, a direct-current power supply 3 which supplies a direct-current power voltage Vdc to the amplifier 2, a high-frequency output detector 4 which detects the high-frequency output that is output by the amplifier 2, and a controller 5 which controls the high-frequency output that is detected by the high-frequency output detector 4 so that this output is maintained at a high-frequency output set value Pfset. The output of the amplifier 2 is supplied to a load 6.
The oscillator 1 is constructed from an oscillator which generates a high-frequency signal that has a specified frequency, and an amplifier which amplifies the output of this oscillator (if necessary). The amplifier 2 is constructed from a power amplifier 2a. 
The controller 5 inputs the high-frequency output set value Pfset and the high-frequency output Pfdet that is detected by the high-frequency output detector 4, and controls the output voltage Vdc of the direct-current power supply 3, the output of the oscillator 1 and the gain of the amplifier 2 so that the high-frequency output Pfout that is supplied to the load 6 from the amplifier 2 is maintained at a value that is equal to the high-frequency output set value Pfset.
In such a high-frequency power supply device, when the matching of the output impedance of the high-frequency power supply device and the load impedance is lost as a result of fluctuations in the load, an excessively large reflected power flows into the amplifier 102 from the side of the load 6, so that there is a danger of damage. Conventionally, therefore, a method has been used (as indicated for example in Japanese Patent Publication No. 5-76045 and Japanese Patent Application Laid-Open No. 2001-244754) in which a protective set value with a magnitude which is such that there is no danger of damage to the amplifier is set for the reflected power, and control that limits the high-frequency output (forward power or effective power) that is supplied to the load from the high-frequency power supply device is performed so that the reflected power from the load does not exceed the protective set value, thus protecting the amplifier from the reflected power.
Furthermore, for example, in Japanese Patent Application Laid-Open No. 11-233294 and Japanese Patent Application Laid-Open No. 2001-35699, high-frequency power supply devices are described in which the loss generated in the amplifier is determined, and the output of the amplifier is controlled so that this loss is maintained at a value that is equal to or less than a loss set value set in the vicinity of the maximum value of the range in which there is no damage to the amplifier.
However, in conventional methods for protecting the amplifier from the reflected power in a high-frequency power supply device, in cases where the loss generated in the amplifier by the reflected power increases so that there is a danger of damage to the amplifier, protection of the amplifier is accomplished by lowering the output of the amplifier. As a result, the following problem arises: namely, when control that protects the amplifier is performed, the power supply output (forward power or effective power) is limited to a value that is considerably lower than the set value.
Especially in cases where the impedance of the load that is connected between the output terminals of the high-frequency power supply device varies, even if the magnitude of the reflection coefficient is the same, the maximum high-frequency output (i.e., the maximum value of the forward power or effective power that is output from the high-frequency power supply device) varies if the phase angle of the reflection coefficient varies. Accordingly, the following problem arises: namely, the maximum high-frequency output is conspicuously reduced with respect to the load impedance at which the phase angle of the reflection coefficient shows a specified value.
Furthermore, if an attempt is made to increase the maximum high-frequency output of the high-frequency power supply device with respect to a load at which the phase angle of the reflection coefficient shows a specified value, the loss generated in the amplifier is increased, so that there is a danger that the amplifier will be damaged by the heat generated by this loss. In concrete terms, the junction temperature of the semiconductor amplifier elements installed in the amplifier exceeds the permissible value, so that there is a danger that these semiconductor amplifier elements will be damaged. As a result, it is substantially impossible to increase the maximum high-frequency output of the high-frequency power supply device with respect to a load at which the phase angle of the reflection coefficient shows a specified value.